The invention relates to a process for the preparation of a catalyst suitable for the conversion of a mixture of carbon monoxide and hydrogen into hydrocarbons, particularly middle distillates.
The preparation of hydrocarbons from a H.sub.2 /CO mixture by contacting this mixture at elevated temperature and pressure with a catalyst is known in the literature as the Fischer-Tropsch hydrocarbon synthesis. Catalyst often used for the purpose comprise one or more metals from the iron group, together with one or more promotors, and a carrier material. These catalysts can suitably be prepared by known techniques, such as precipitation, impregnation, kneading and melting. The products which can be prepared by using these catalysts usually have a very wide range of molecular weight distributions and, in addition to branched and unbranched paraffins, often contain considerable amounts of olefins and oxygen-containing organic compounds. Usually only a minor portion of the products obtained is made up of middle distillates. Of these middle distillates not only the yield but also the pour point is unsatisfactory. Therefore the direct conversion of H.sub.2 /CO mixtures according to Fischer-Tropsch is not a very attractive route for the production of middle distillates on a technical scale.
In this patent application "middle distillates" should be taken to be hydrocarbon mixtures whose boiling range corresponds substantially with that of the kerosene and gas oil fractions obtained in the conventional atmospheric distillation of crude mineral oil. During said distillation, from the crude mineral oil are separated in succession: one or more gasoline fractions having a boiling range between 30.degree. C. and 200.degree. C., one or more kerosene fractions having a boiling range between 140.degree. and 300.degree. C. and one or more gas oil fractions having a boiling range between 180.degree. and 370.degree. C.
Recently a class of Fischer-Tropsch catalysts was found which have the property of yielding a product in which only very few olefins and oxygen-containing compounds occur and which consists virtually completely of unbranched paraffins, a considerable portion of which paraffins boils above the middle distillate range. It has been found that by using a catalytic hydrotreatment this product can be converted in high yield into middle distillates. As feed for the hydrotreatment at least the part of the product is chosen whose initial boiling point lies above the final boiling point of the heaviest middle distillate desired as end product. The hydrotreatment, which is characterized by a very low hydrogen consumption, leads to middle distillates with a considerably better pour point than those obtained in the direct conversion of a H.sub.2 /CO mixture according to Fischer-Tropsch.
The Fischer-Tropsch catalysts belonging to the above-mentioned class contain silica, alumina or silica-alumina as carrier material and cobalt together with zirconium, titanium and/or chromium as catalytically active metals, in such quantities that the catalysts comprise 3-60 pbw cobalt and 0.1-100 pbw of at least one other metal chosen from zirconium, titanium or chromium per 100 pbw carrier material. The catalysts are prepared by depositing the metals involved on the carrier material by kneading and/or impregnation.
The catalyst preparation by impregnation is carried out by contacting a porous carrier with a compound of the relevant metal in the presence of a liquid, followed by removal of the liquid. Likewise, in the catalyst preparation by kneading a porous carrier is contacted with a compound of the relevant metal in the presence of a liquid, followed by removal of the liquid, the distinction being that preceding and/or during the removal of the liquid the composition is subjected to intensive mechanical treatment, such as pressing, kneading or wringing, which as a rule leads to a considerable reduction of the particle size of the carrier material and to the composition taking on the consistency of a paste. Usually several hours' kneading in a suitable kneading machine is sufficient to attain the desired homogeneity of distribution of the components over the mixture. The intensive mechanical treatment during which there is a considerable reduction of the particle size of the carrier material is the main difference between the kneading route and the impregnation route. It is true that during the preparation of a catalyst by impregnation there may be a stage at which the composition contains an amount of liquid corresponding to that present in the paste mentioned hereinbefore and that some mechanical energy may be supplied to the composition, for instance by stirring, but on the whole the particle size of the carrier material will remain virtually unchanged during the catalyst preparation by impregnation.
In the case of the present catalysts which in addition to cobalt comprise another metal, the term "catalyst preparation by kneading and/or impregnation" as used herein includes every possible variant in which the two metals have been deposited on the carrier in this way. For instance, the cobalt may be deposited on the carrier entirely by kneading and the other metal entirely be impregnation, but it is also possible to deposit part of the cobalt and/or the other metal by impregnation, while the rest is deposited by kneading; Likewise, there is a free choice as to the order in which the metals are deposited on the carrier and the number of steps used to carry out the catalyst preparation.